As Lisa Jackson gets ready to step down as head of the EPA shortly after the president's inauguration later this month, she is being hailed by environmentalists for pushing through the toughest new air and water pollution rules in over two decades, and speaking out on climate change in an administration that has largely avoided confronting the issue head-on.

But not everybody is sad to see the EPA administrator go. During the recent election campaign, Mitt Romney called for Jackson's resignation, and some Republicans in Congress have accused her of waging a "war on coal". Jackson's EPA drafted regulations that limit mercury emissions from coal-fired power plants. She also initiated a sweeping agency review of the impact of mountaintop removal in states like West Virginia and Kentucky, a practice that dumps raw mining waste into wetlands and streams.

Jackson, the nation's first black EPA chief, grew up as the child of a postal worker in New Orleans. As a Louisiana native, she was the public face of the administration's responses to the massive Deepwater Horizon oil spill in the Gulf in 2010, hosting town meetings throughout the region to reassure residents of the government's support.

Jackson often spoke about the fact that the nation's poor frequently live in industrial zones where they suffer disproportionally from pollution of the air and water. The health impacts of pollution hit close to home for Jackson. She spoke publicly of the anguish she felt watching her infant son suffer from asthma, an illness associated with high levels of particulates in the air.

In a phone interview, Tom "Smitty" Smith, the Texas director of Public Citizen, told me that Jackson was the first EPA administrator in three decades to seek out the views of local environmentalists during her trips to Texas, and not just politicians and representatives of the oil industry. As a result of these meetings, Jackson closed loopholes in the permitting process which had allowed refineries on what Smith calls "the cancer coast" between Port Arthur and Corpus Christi to employ inferior pollution control technologies.

Emissions have gone down as much as 50% at some Texas plants. Smith calls Jackson, "Smart, tough, competent: probably the best administrator that the EPA has ever had."

The president has not yet chosen a successor at the EPA. Speculation has focused on two of Jackson's subordinates, Bob Perciasepe, the agency's deputy administrator, who was formerly the chief operating officer at the National Audubon Society, and Gina McCarthy, an assistant administrator for the Office of Air and Radiation, who often filled in for Jackson at congressional committee hearings.

Whoever Obama chooses will face resistance from the Republican-controlled House. Bob Dean, a spokesman at the Natural Resources Defense Council, told me that the biggest challenge for the next EPA chief will come from mostly Republican legislators who hold "entrenched anti-environment position on ideological grounds". In contrast to the nearly 40 years of bipartisan cooperation on the environment, Dean says recent years have seen "one vote after another to undermine and delay needed regulations".

Smith of Public Citizen worries, in particular, about the House Appropriations Committee, which currently has a 21-to-14 Republican majority. Republicans, using the fiscal cliff and the need for a balanced budget as a pretext, might try to block critical funds required by the EPA to enforce its own regulations.

When I asked the environmentalists whom I interviewed their priority for the EPA during this second Obama administration, there was surprising agreement. All put cutting carbon pollution from power plants at the top of their list. The Environmental Defense Fund's senior vice-president, Eric Pooley, spoke to this consensus when he applauded the Obama administration for enacting tough regulations on newly-built power plants to cut C02 emissions, but added:

"It's time to extend these vital safeguards to existing power plants as well. Doing so would be a huge step forward on the road to confronting global climate change."

In addition to cutting greenhouse gases, environmentalist are hoping that the new administration will continue to block the Keystone XL pipeline, which would transport vast quantities of "dirty oil" from northern Canada to the US Gulf coast. The president has said that he will make a final decision on Keystone during the current year.

There has been speculation that Jackson is resigning because the president has already decided to approve the controversial pipeline. Jackson "left as a matter of conscience," according to Jeff Tittel, the director of New Jersey's Sierra Club chapter and a longtime friend of the EPA administrator. Tittel says she "has too much principle to support [the pipeline], between the climate impacts of it and the water quality impacts of it … She was the person who pushed the hardest for the moratorium on the pipeline and now she's leaving."

Jackson herself says publicly only that the time has come to move on. Environmental activists hope that whoever replaces her will have Jackson's backbone in pushing forward on the gains she initiated during her controversial and courageous four-year tenure.